Appendix 4.3 Determination
of Emission Rates for Odour Impact Assessment
In order to determine the average rate at which odour is released into the atmosphere as a result of the activities carried out within the proposed Wholesale Fish Market (WFM), odour testings were conducted at the existing Castle Peak Fish Market (CPFM) at Sam Shing Estate. Details of the odour testing is described below.
Odour Testing at the Castle Peak Fish Market
In view of the fact that the proposed WFM is to replace the existing temporary CPFM at Sam Shing Estate in Tuen Mun, the activities at the existing CPFM is considered representative of the proposed one. In addition, as confirmed by the AFCD, the design capacity as well as the peak intensity of activity at the proposed WFM is similar to that of the existing CPFM, the potential of odour emission for the two would be alike. Measurements of the odour concentration within the CPFM were conducted in order to determine the emission rate to be used in the prediction of odour impact due to the operation of the proposed WFM on the nearby sensitive receivers.
The characteristics of the odour found at the existing CPFM are that typical of chilled fish. Since the CPFM is located at the coast of the Sam Shing Estate, the smell is not particularly distinguished from the ambient odour environment which is characterized by the smell of the sea.
The configuration of the existing CPFM is similar to that of the proposed one, with the sorting, marshalling and auction areas being the major sources of odour emission. The major difference is that there is no designated area for lorry parking and loading of fish. Currently, some of these activities are carried out on the road outside the fish market. Moreover, there are many openings at high levels on the walls at the existing CPFM. These openings can facilitate the flow of air in and out of the fish market by means of natural ventilation. However, they also promote the escape of odourous airborne substances to the surrounding environment.
Upon site investigation to the existing CPFM, it was found that the odour strength at the marshalling and auction area is higher than that at the sorting area. This can be attributed to the fact that the chilled fish, while displayed for sale or awaited to be loaded onto lorries, usually stay longer at the marshalling and auction area. In addition, the sorting area is situated along the sea front where the movement of air is generally greater and any odorous compounds emitted from the fish stocks are dispersed before significant odour concentration can build up.
Odour testing was conducted at the CPFM on 25 August 2001 in order to study the worst-case odour emission strength during summer time. Two air samples were taken at different heights of the same location inside the sorting and marshalling area where the fishy smell was most strong. The sampling was carried out during peak-hour of the CPFM’s operation when the marshalling and auction area was filled with containers of fish and other seafood waiting to be auctioned or loaded onto trucks and taken away.
The odour sampling was conducted in accordance with the method recommended in “Sampling Techniques for Olfactometric Analysis” published by The University of New South Wales[1]. The odour sampling procedure is briefly described as follows. Odour sampling was carried out using an air sampler which consists of an air bag, a plastic barrel, a vacuum pump and a battery. The air-sampling bag was placed inside a barrel, which was connected to an air intake tube and a vacuum driver. The air intake tube was positioned at appropriate heights at the designated locations. When the barrel was sealed and the vacuum pump was switched on, the ambient air was sucked into the air-sampling bag until it was full. The air sampling bag was then properly sealed and delivered immediately to the Odour Research Laboratory of the Hong Kong Polytechnic University for olfactometry testing.
Odour samples were taken with an air sampler at the designated measurement locations. The odour concentration inside the fish market varies temporally and spatially. The existing CPFM relies on natural ventilation and at the time of sampling, there was no mechanical equipment to enhance the air mixing within the fish market. To ensure that the samples represent the worst-case odour concentration within the fish market, the samples were taken at locations where the fishy smell was strongest. The sampler was placed directly above the odour source with intake locations at 1.5m and 3m above ground. As there was little air movement inside the CPFM, the measured odour concentrations is considered representative of that of the stable layers of air parcels located at different heights inside the CPFM.
The olfactometry test was conducted at the Odour Research Laboratory of the Hong Kong Polytechnic University. The odour concentration of the air samples was determined using a Forced-choice Dynamic Olfactometer in accordance with the Dutch National Standard (NVN 2820). In the test, seven qualified panellists, who were previously selected through screening tests by using a 50ppm certified n-butanol gas as a standard sample, participated in the odour testing sessions. Odour concentration was determined based upon the olfactory responses of the panellists sniffing diluted odour samples presented by an olfactometer. The odour testing results are summarized below and given in Annex A. The testing results indicate that the odour concentration decrease with height above the odour source.
|
Sampling
Particulars |
Odour
Concentration |
||
|
Date |
Time |
Height above
ground (m) |
(OU/m3) |
|
25/8/01 |
04:30 |
1.5 |
432 |
|
25/8/01 |
04:45 |
3.0 |
279 |
Estimation of Odour Emission Rate
Since the operation of the proposed WFM will resemble that of the existing CPFM, the measured odour strength is considered representative of the peak-hour operation of the future WFM. The rate of odour emission from the proposed WFM is estimated based on the measured odour concentration as described below.
Firstly, the average odour concentration inside the WFM is determined as follows :
Assuming the variation of odour concentration with height following Power Law,
measured odour concentrations at the marshalling and auction area of the CPFM,
odour concentration c1 = 432OU/m3 at h1=1.5m
odour concentration c2 = 279OU/m3 at h2=3m
Therefore, the relationship between indoor odour concentration (c’) and height above ground (h) is given by
c’=432(h’/1.5)^-0.63 OU/m3
Average odour concentration inside the sorting, marshalling and auction area (Cavg) can be estimated using,
OU/m3
where
c = odour concentration as a function of height above ground in OU/m3;
A = area of odour source = 1440m2 (assuming 50% of the floor area is covered with fish load);
V = volume of sorting, marshalling and auction area = 18146.25m3 (please refer to Figures A4.3-1 & A4.3-2 for details);
h = height above ground in m; h1 = 0m, h2 = 5.5m
As observed during visits to the existing CPFM on various occasions, including visits on 16 November 2000 and 25 August 2001, the sorting, marshalling and auction area in the CPFM was generally filled with patches of containers holding the fish stocks (see Figure A4.3-3). The area covered by these fish stocks was generally less than a half of the total floor area, leaving space for access, storage area and room for transport of the fish stocks as well as other operational activities, including weighing of fish. In the current study, it is assumed that the total area of the odour source covers 50% of the total floor space of the sorting, marshalling and auction area as a worst-case scenario. The average odour concentration (Cavg) inside the WFM during its peak-hour operation is estimated to be 225OU/m3.
Secondly, the rate of odour emission from the WFM is derived from the average odour concentration determined. In estimating the odour emission rate due to the WFM’s operation, it was conservatively assumed that all of the odorous air inside the sorting, marshalling and auction area will be emitted to the surrounding environment and the mechanism for this process will be by means of mechanical ventilation.
A schematic diagram of the proposed mechanical ventilation system for the WFM is shown in Figure A4.3-4. Information on the preliminary design of mechanical ventilation system for the WFM is given below :
- a minimum of 10 air change/hour will be maintained throughout the WFM;
- estimated ventilation rate for sorting, marshalling and auction area is 50.4m3/s;
- estimated ventilation rate for the loading/parking area is 47.7m3/s;
- fresh air intake will be provided through vent shaft located on the landscaped deck or controlled openings on walls along the western site boundary; and
- a continuous rectangular extracted air discharge with surface area of 80m´0.5m will be installed at 3.3m above ground level. It will be located near and directed towards the seaside.
The odour concentration in the extracted air is estimated as follows :
Assuming the air volume inside the sorting, marshalling and auction area is in a well-mixed condition with mechanical ventilation, the average odour concentration is diluted by 10 times according to the air change rate.
Average odour concentration inside the sorting, marshalling and auction area with mechanical ventilation = 225/10 = 22.5 OU/m3
The odorous air extracted from the sorting, marshalling and auction area is further diluted by the relatively non-odorous air extracted from the loading/parking area,
Odour concentration in extracted air = 22.5´50.4/98.1 = 11.56OU/m3
The rate of odour emission from the WFM during its peak-hour operation
= Odour concentration in extracted air ´ total ventilation rate
= 11.56OU/m3 ´ 98.1m3/s
= 1134OU/s.